Method of transmitting data in wireless communication system

ABSTRACT

The method includes generating coded bits by encoding information bits, dividing the coded bits into a first bit-stream and a second bit-stream, generating a first data symbol by performing anti-gray mapping on the first bit-stream, generating a second data symbol by performing gray mapping on the second bit-stream, and transmitting the first data symbol and the second data symbol.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a National Stage filing under 35 U.S.C. §371 ofInternational Application PCT/KR2008/004381, filed on Jul. 28, 2008,which claims the benefit of earlier filing date and right of priority toKorean Application No. 10-2007-0078654, filed on Aug. 6, 2007.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a method of transmitting data for improving reliabilityof bits in a wireless communication system.

BACKGROUND ART

Digital signals are transmitted through various propagation paths in awireless communication system. Further, the digital signals arereproduced from a recording medium such as a compact disc (CD) or adigital versatile disc (DVD). When the digital signals are transmittedand reproduced through various channels, various data errors may becaused due to noise and variations.

In order to overcome a data error, an error correction code can be used.The error correction code is a code in which each data signal conformsto specific rules of construction so that information bits in thereceived signal can generally be automatically detected and corrected.

A turbo code is one of error correction codes used in many wirelesscommunication systems. The turbo code has a relatively simple decodingalgorithm and also has a significantly low bit error rate. Basically,the turbo code is a parallel concatenated convolution code. In general,the turbo code applies different arrays of the same bit-stream to aconstituent encoder. That is, the same bit-stream is used in theconstituent encoder by changing only bit arrangement.

For decoding, the turbo code uses a soft-output iterative decodingscheme. A Viterbi algorithm has generally been used in the convolutionalcode. Although it is an optimal method for minimizing a bit-streamerror, bit-unit information cannot be generated with the Viterbialgorithm. In the decoding process, soft-output information of each bitis exchanged, and the exchanged information is used in a next decodingprocess, thereby improving performance.

A data transmission/reception process is generally performed in atypical communication system in the following manner. When informationbits are input, channel encoding is performed to output coded bits. Thecoded bits are interleaved and then mapped in a unit of symbols. Themapped symbols are modulated and then transmitted. The symbolstransmitted through a channel are de-mapped by a de-mapper. Thede-mapped bit information is de-interleaved and then channel-decoded.The channel decoder reports a reliability level of a coded bit and anestimated data bit. The reliability level of the coded bit isinterleaved and then input to the de-mapper. This information allows thede-mapper to output improved bit information. The improved bitinformation is de-interleaved and then input to the channel decoder. Inan iterative decoding process, the de-mapper and the channel decoderiterate de-mapping and decoding functions a predetermined number oftimes while improving the mutual functions, and obtained information onthe estimated data bit is finally estimated in a next iteration. Throughthe iterative decoding, the channel decoder can output the reliabilitylevel of the coded bit and the estimated data bit.

Accordingly, in the communication system using the turbo code, decodingperformance can be improved by improving reliability of coded bits.

DISCLOSURE OF INVENTION Technical Problem

A method is sought for improving reliability of coded bits by mappingthe coded bits according to different mapping schemes.

Technical Solution

In an aspect, a method of transmitting data in a wireless communicationsystem is provided. The method includes generating coded bits byencoding information bits, dividing the coded bits into a firstbit-stream and a second bit-stream, generating a first data symbol byperforming anti-gray mapping on the first bit-stream, generating asecond data symbol by performing gray mapping on the second bit-stream,and transmitting the first data symbol and the second data symbol.

The coded bits can be generated by using a turbo code. The firstbit-stream can be systematic bits and the second bit-stream can beparity bits.

In another aspect, a transmitter includes a channel encoder configuredto generate coded bits by encoding information bits, and a mappercomprising a first mapping unit which maps systematic bits among thecoded bits by using a first mapping scheme to generate a first datasymbol, and a second mapping unit which maps parity bits among the codedbits by using a second mapping scheme to generate a second data symbol.

Advantageous Effects

After channel encoding is performed using a turbo code, reliability isimproved both in systematic bits and parity bits. Detection performancecan be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 is a block diagram showing a transmitter according to anembodiment of the present invention.

FIG. 3 shows an example of a channel encoder.

FIG. 4 is a block diagram showing a bit separation unit and a mapper.

FIG. 5 shows an example of gray mapping in 8-phase shift keying.

FIG. 6 shows an example of anti-gray mapping.

FIG. 7 is a block diagram showing a receiver according to an embodimentof the present invention.

MODE FOR THE INVENTION

FIG. 1 shows a wireless communication system. The wireless communicationsystem can be widely deployed to provide a variety of communicationservices, such as voices, packet data, etc.

Referring to FIG. 1, a wireless communication system includes at leastone user equipment (UE) 10 and a base station (BS) 20. The UE 10 may befixed or mobile, and may be referred to as another terminology, such asa mobile station (MS), a user terminal (UT), a subscriber station (SS),a wireless device, etc. The BS 20 is generally a fixed station thatcommunicates with the UE 10 and may be referred to as anotherterminology, such as a node-B, a base transceiver system (BTS), anaccess point, etc. There are one or more cells within the coverage ofthe BS 20.

Hereinafter, a downlink is defined as a communication link from the BS20 to the UE 10, and an uplink is defined as a communication link fromthe UE 10 to the BS 20. In the downlink, a transmitter may be a part ofthe BS 20, and a receiver may be a part of the UE 10. In the uplink, thetransmitter may be a part of the UE 10, and the receiver may be a partof the BS 20.

The wireless communication system may be an orthogonal frequencydivision multiplexing (OFDM)/orthogonal frequency division multipleaccess (OFDMA)-based system. The OFDM uses a plurality of orthogonalsubcarriers. Further, the OFDM uses an orthogonality between inversefast Fourier transform (IFFT) and fast Fourier transform (FFT). Thetransmitter transmits data by performing IFFT. The receiver restoresoriginal data by performing FFT on a received signal. The transmitteruses IFFT to combine the plurality of subcarriers, and the receiver usesFFT to split the plurality of subcarriers.

The communication system may have one or a plurality of transmit (Tx)antennas. The communication system may be a multiple-inputmultiple-output (MIMO) system, a multiple-input single-output (MISO)system, a single-input single-output (SISO) system, or a single-inputmultiple-output (SIMO) system. The MIMO system uses a plurality of Txantennas and a plurality of receive (Rx) antennas. The MISO system usesa plurality of Tx antennas and one Rx antenna. The SISO system uses oneTx antenna and one Rx antenna. The SIMO system uses one Tx antenna and aplurality of Rx antennas.

FIG. 2 is a block diagram showing a transmitter according to anembodiment of the present invention.

Referring to FIG. 2, a transmitter 100 includes a channel encoder 110, abit interleaver 120, a serial-to-parallel (S/P) converter 130, a bitseparation unit 140, a mapper 150, a MIMO precoder 160 and an IFFT unit170. The transmitter 100 also includes a plurality of Tx antennas 190.

The channel encoder 110 performs channel encoding on information bitsand outputs coded bits. A turbo-type code can be used in the channelencoding.

FIG. 3 shows an example of the channel encoder. The channel encoder 110is a convolutional turbo code (CTC) encoder. The section 8.3.3.2.3 ofthe institute of electrical and electronics engineers (IEEE) 802.16-2004standard may be incorporated herein by reference.

Referring to FIG. 3, the channel encoder 110 includes a CTC interleaver112, a constituent encoder 114, and a puncturing unit 116. The channelencoder 110 uses a double binary circular recursive systematicconvolutional code. Information bits A and B are input and then encodedby the constituent encoder 114. This encoding process is referred to asC₁ encoding. The information bits are interleaved by the CTC interleaver112 and then encoded by the constituent encoder 114. This encodingprocess is referred to as C₂ encoding.

Two types of bit-streams, i.e., systematic bits and parity bits, can beoutput from the channel encoder 110. When input to the channel encoder110, systematic bits (indicated by A or B in FIG. 3) are directly outputwithout further processing. The parity bits (indicated by Y₁ or Y₂ inFIG. 3) are encoded by the constituent encoder 114.

The structure of channel encoder 110 is shown for exemplary purposesonly, and thus the channel encoder 110 may use other turbo-type codes.For example, a channel encoder used in a universal mobiletelecommunications system (UMTS) of the 3rd generation partnershipproject (3GPP) may be implemented by incorporating the section 4.2.3.2in the 3GPP TS 25.212 V7.1.0 (2006-06) entitled “Multiplexing andchannel coding (FDD) (Release 7)” by reference.

Referring back to FIG. 2, the bit interleaver 120 interleaves the codedbits in a bit unit. The bit interleaver 120 spreads centralized errorsoccurring in a channel, and thus obtains a diversity gain in a bit unit.The S/P converter 130 converts a serial sequence of the interleavedbit-stream into a parallel sequence.

The bit separation unit 140 separates the input bit-stream intosystematic bits (i.e., a first bit-stream) and parity bits (i.e., asecond bit-stream). If the systematic bits are interleaved together withthe parity bits, the bit separation unit 140 can separate the inputbit-stream into the systematic bits and the parity bits according to aninterleaving pattern. If the systematic bits are interleaved separatelyfrom the parity bits, the bit separation unit 140 can simply separatethe input bit-stream into the systematic bits and the parity bits.Alternatively, instead of the bit interleaver 120, a symbol interleavermay be disposed next to the mapper 150. In this case, a plurality ofpieces of information separated by the bit separation unit 140 can berespectively transmitted via the symbol interleaver.

By applying different mapping schemes to the systematic bits (i.e., thefirst bit-stream) and the parity bits (i.e., the second bit-stream), themapper 150 maps the bits onto data symbols. When symbols of a specificbit-stream are represented with locations on a signal constellation,these symbols are referred to as the data symbol. To improve reliabilityof the bits in iterative decoding, the mapper 150 applies a firstmapping scheme (e.g., anti-gray mapping) to the systematic bits andapplies a second mapping scheme (e.g., gray mapping) to the parity bits.

The MIMO precoder 160 performs MIMO precoding on input data symbols. TheMIMO precoder 160 may use a space-time block code (STBC). For example,if it is assumed that 4 Tx antennas 190 are used and a ¾ orthogonal STBCstructure is selected, the STBC can be expressed as shown:

$\begin{matrix}{{{MathFigure}\mspace{14mu} 1}\mspace{526mu}} & \; \\{C = \begin{bmatrix}z_{1} & z_{2} & z_{3} & 0 \\{- z_{2}^{*}} & z_{1}^{*} & 0 & {- z_{3}} \\{- z_{3}^{*}} & 0 & z_{1}^{*} & z_{2} \\0 & z_{3}^{*} & {- z_{2}^{*}} & z_{1}\end{bmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack\end{matrix}$

where z_(j) denotes a j-th symbol signal transmitted by the transmitter.When the above STBC coding is performed, 3 symbol signals aretransmitted for 4 time slots.

The IFFT unit 170 performs an IFFT on input symbols and thus outputsOFDM symbols. The OFDM symbols are transmitted through the Tx antenna190.

FIG. 4 is a block diagram showing the bit separation unit and themapper.

Referring to FIG. 4, the mapper 150 includes a first mapping unit 150 aand a second mapping unit 150 b. The first mapping unit 150 a performsanti-gray mapping. The second mapping unit 150 b performs gray mapping.The bit separation unit 140 separates an input bit-stream intosystematic bits and parity bits. The systematic bits are input to thefirst mapping unit 150 a and then subjected to the anti-gray mapping.The parity bits are input to the second mapping unit 150 b and thensubjected to the gray mapping.

FIG. 5 shows an example of gray mapping in 8-phase shift keying (PSK).

Referring to FIG. 5, when adjacent symbols are mapped with the graymapping, bits representing one symbol are different by only one bit frombits representing the other symbol. For example, if a symbol P1 isrepresented with bits ‘011’, two symbols P2 and P3 adjacent to thesymbol P1 are respectively represented with bits ‘001’ and ‘010’, eachof which is different by one bit from the bits representing the symbolP1.

FIG. 6 shows an example of anti-gray mapping.

Referring to FIG. 6, when adjacent symbols are mapped with the anti-graymapping, bits representing one symbol are not necessarily different byonly one bit from bits representing the other symbol. That is, one ormore bits may be different. For example, if a symbol Q1 is representedwith bits ‘010’, two symbols Q2 and Q3 adjacent to the symbol Q1 arerespectively represented with bits ‘101’ and ‘111’, which arerespectively different by three bits and 2 bits from the bitsrepresenting the symbol Q1.

Although 8-PSK is shown as an example, quadrature phase shift keying(QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM, 245-QAM, etc.,may also be used in the present invention. In addition, bitsrepresenting each symbol are shown for exemplary purposes only, and thusvarious modifications can be made therein as long as requirements of thegray mapping or the anti-gray mapping are satisfied.

When a turbo code is used in iterative decoding, entire systemperformance can be improved by selecting a suitable mapping scheme. Themapping scheme may use a technique disclosed in the document entitled“Mapping Optimization for Space-Time Bit-Interleaved Coded ModulationWith Iterative Decoding” (IEEE Transactions on communications, Vol. 55,No. 4, April 2007 by Wookbong Lee, Jungho Cho, ChangKyung Sung, HwangjunSong, and Inkyu Lee). Gray mapping or anti-gray mapping may be usedwithout distinguishing systematic bits and parity bits in a process ofmapping bit-unit information onto symbols on a signal constellation. Inthis case, only priori values for the systematic bits are extracted andupdated in the iterative decoding process. Thus, a reliability level ofthe systematic bits can be increased through the iterative decodingprocess, whereas the reliability level of the parity bits is notincreased. The gray mapping outperforms the anti-gray mapping when theiterative decoding process is not performed. On the other hand, when theiterative decoding process is performed, the anti-gray mappingexperiences performance improvement while the gray mapping experiencesnearly no improvement in performance.

When the iterative decoding is used, performance may deteriorate if theanti-gray mapping is applied to the parity bits whose reliability levelis not increased. Performance can be improved when the anti-gray mappingis applied to the systematic bits whose reliability level can beincreased through the iterative decoding and when the gray mapping isapplied to the parity bits.

FIG. 7 is a block diagram showing a receiver according to an embodimentof the present invention.

Referring to FIG. 7, a receiver 200 includes an FFT unit 210, a MIMOpost-coder 220, a de-mapper 230, a parallel-to-serial (P/S) converter240, a bit de-interleaver 250, a decoder 260, a bit interleaver 270, andan S/P converter 280. The FFT unit 210 performs an FFT on a signalreceived from an Rx antenna 290 and outputs a frequency-domain signal.The MIMO post-processor 220 performs post-processing corresponding tothe precoding of the MIMO precoder 160. The de-mapper 230 performsde-mapping on the frequency-domain signal and outputs a soft-outputvalue for estimated bits.

The P/S converter 240 converts a parallel sequence of a bit-stream intoa serial sequence. The de-interleaver 250 de-interleaves bits so thatthe bits are arranged in the same order as before the order of bits areinterleaved by the interleaver 120.

The decoder 260 outputs probability information for all received bitsignals. The decoder 260 estimates a probability that each data bit ofan input bit-stream is ‘1’ or ‘0’, and thus generates the probabilityinformation. The bit interleaver 270 changes a bit order of thebit-stream. The S/P converter 280 converts a serial sequence of thebit-stream into a parallel sequence, and delivers the parallel sequenceto each de-mapper 230. The de-mapper 230 performs de-mapping byappending the probability information to an original signal. Theprobability information for each data bit is generated and used so thatde-mapping is performed a predetermined number of times on each databit. This process is referred to as an iterative decoding process.

The probability information of a data bit input to the decoder 260 is asfollows. The probability information indicates a reliability level ofmapping when input symbol information is mapped. A probability valueused herein indicates a probability that the input symbol information is‘+1’ or ‘−1’.

$\begin{matrix}{{{MathFigure}\mspace{14mu} 2}\mspace{526mu}} & \; \\{{L\left( d_{j}^{i} \right)} = {\log\frac{p\left( {d_{j}^{i} = {+ 1}} \right)}{p\left( {d_{j}^{i} = {- 1}} \right)}}} & \left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Equation 2, d^(i) _(j) denotes information corresponding to an i-thbit of a j-th symbol. If a probability value p(d^(i) _(j)=+1) denotes aprobability that a symbol vector value corresponding to a bit valueinput to the decoder 260 is ‘+1’ and if a probability value p(d^(i)_(j)=−1) denotes a probability that the above symbol vector value is‘−1’, then a ratio of the two probability values is obtained so that alog value of the ratio is defined as probability information provided toeach de-mapper 230.

The probability information generated by the decoder 260 is input to thebit interleaver 270. The bit interleaver 270 changes a bit order of thereceived probability information according to a method used when theorders of the bit-stream are interleaved by the bit interleaver 120 ofthe transmitter 100. Probability information of each signal is deliveredto the de-mapper 230. By appending the received probability informationto the previous received symbol signal, the de-mapper 230 can increasean accuracy level of the symbol value. The greater the accuracy level,the more accurately the original bit signal can be restored.

$\begin{matrix}{{{MathFigure}\mspace{14mu} 3}\mspace{526mu}} & \; \\{{\log\frac{p\left( {{d_{j}^{i} = {{+ 1}❘r_{j}}},H_{j}} \right)}{p\left( {{d_{j}^{i} = {{- 1}❘r_{j}}},H_{j}} \right)}} = {\log\frac{\sum\limits_{z_{j} \in S_{+ 1}^{i}}^{\;}\;{p\left( {z_{j},r_{j},H_{j}} \right)}}{\sum\limits_{z_{j} \in S_{- 1}^{i}}^{\;}\;{p\left( {z_{j},r_{j},H_{j}} \right)}}}} & \left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack\end{matrix}$

Equation 3 is an example of calculating the probability informationvalue. The probability information value is calculated by substitutingchannel state information to Equation 1 above. In Equation 3, Siddenotes a set of symbol vectors in which an i-th bit is set to ‘d’. Thevalue ‘d’ may be ‘+1’ or ‘−1’.

The de-mapping and the decoding are repeated a predetermined number oftimes. While repeating this process, the probability information valueis updated whenever the process is iterated so that the probabilityinformation value includes a value calculated through a previousiteration. The probability information value updated by Equation 3 canbe generated according to Equation 4 below.

$\begin{matrix}{{{MathFigure}\mspace{14mu} 4}\mspace{526mu}} & \; \\{{p\left( {z_{j},r_{j},H_{j}} \right)} \sim {\exp\left( {{{- \frac{1}{N_{o}}}{{r_{j} - {\alpha\; z_{j}}}}^{2}} + {\frac{1}{2}{\sum\limits_{i = 1}^{\log_{2}M}{d_{j}^{i}{L\left( d_{j}^{i} \right)}}}}} \right)}} & \left\lbrack {{Math}.\mspace{14mu} 4} \right\rbrack\end{matrix}$

In Equation 4, z_(j) denotes a j-th symbol signal transmitted by thetransmitter, r_(j) denotes a j-th symbol signal received by thereceiver, and Hj denotes a channel constant when a signal is receivedthrough a j-th Rx antenna. In addition, M denotes a size ofconstellation mapping, and N_(o) denotes complex noise power.

Equation 4 above includes only a calculation result of Equation 2 above.That is, the calculation result of Equation 2 is substituted toEquations 3 and 4 to generate the probability information. As a result,a transmitted bit signal can be estimated more accurately. By using thede-mapping result, the probability information is re-calculated, andde-mapping is re-performed using the probability information. Byrepeating this process, an absolute value of an estimated data bit ratiois increased and thus a data bit can be determined more accurately. Assuch, the estimated data bit is finally estimated after iterating aprocess of estimating the data bit a predetermined number of times.

After channel encoding is performed using a turbo code, reliability isimproved both in systematic bits and parity bits. Therefore, entiresystem performance can be improved.

The technique described above can be used in uplink transmission and/ordownlink transmission. Since a data rate can be increased whenreliability of bits is improved, the technique may be more effectivelyapplied in uplink transmission sensitively affected by battery capacity.

Every function as described above can be performed by a processor suchas a microprocessor based on software coded to perform such function, aprogram code, etc., a controller, a micro-controller, an ASIC(Application Specific Integrated Circuit), or the like. Planning,developing and implementing such codes may be obvious for the skilledperson in the art based on the description of the present invention.

Although the embodiments of the present invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope of the invention. Accordingly, the embodimentsof the present invention are not limited to the above-describedembodiments but are defined by the claims which follow, along with theirfull scope of equivalents.

The invention claimed is:
 1. A transmitter comprising: a channel encoderconfigured to encode information bits to generate coded bits; a bitseparation unit configured to separate the coded bits into systematicbits and parity bits; and a mapper comprising: a first mapping unitconfigured to map the systematic bits among the coded bits according toan anti-gray mapping scheme to generate a first data symbol; a secondmapping unit configured to map parity bits among the coded bitsaccording to a gray mapping scheme to generate a second data symbol; anda multiple-input multiple-output (MIMO) precoder configured to performMIMO precoding on the first data symbol and the second data symbol usinga space-time block code (STBC) which applies ¾ orthogonal STBCstructure, wherein the STBC is expressed as follows:${C = \begin{bmatrix}z_{1} & z_{2} & z_{3} & 0 \\{- z_{2}^{*}} & z_{1}^{*} & 0 & {- z_{3}} \\{- z_{3}^{*}} & 0 & z_{1}^{*} & z_{2} \\0 & z_{3}^{*} & {- z_{2}} & z_{1}\end{bmatrix}},$ where z_(j) denotes a j-th symbol signal transmitted.2. The transmitter of claim 1, wherein the channel encoder is furtherconfigured to encode the information bits according to a turbo code.